This is best explained by comparing mobile phone handsets and wifi, which operate on different frequencies. Depending on the network provider, the power dissipated by the radio of a mobile phone is no more than 2 watts (O2 and Vodafone) or 1 watt (Orange).
Now consider that the legal limit for wireless networking is 0.1w. It makes one wireless network card equivalent to either 10th or a 20th of that of a mobile phone (depending on the network in question). A mobile phone is generally in contact with your body (call it 1cm). The antenna on your roof is around 5 metres away and is seriously weakened by the building walls and roof.
Furthermore, the radiation from an aerial is subject to the inverse square law. This means that the strength of the radiation decreases proportionally according to the square of the distance away from it. So if you stand twice as far away it’s not half, but a quarter of the strength; three times the distance and it’s nines times weaker. All this means that at 5 metres away the radiation from a wireless node is 2.5 to 5 million times weaker than that from your mobile phone. This doesn’t even take into account the roof and walls in the way which weaken the signal from your node, not to mention the antenna which tries to keep radiation away from your house.
The output power of wireless networks like Bristol Wireless is very low (0.1 watt) and, as the equipment is not held against the user’s head, there is very little exposure to radio wave energy.
A study carried out by the National Radiological Protection Board (NRPB) for the Office of Government Commerce (OGC) found that at times there can be greater background radio intrusion from television and radio transmissions than from wireless network access points.
Manufacturers of wireless network equipment design their products to operate within the safety guidelines created by the NRPB; for further information please visit the Health Protection Agency website.